Continuous Replenishment Crystal Growth

ABSTRACT

An apparatus for growing a crystal includes a growth chamber and a melt chamber thermally isolated from the growth chamber. The growth chamber includes: a growth crucible configured to contain a liquid melt; and a die located in the growth crucible, the die having a die opening and one or more capillaries extending from within the growth crucible toward the die opening. The melt chamber includes: a melt crucible configured to receive feedstock material; and at least one heating element positioned within the melt chamber relative to the melt crucible to melt the feedstock material within the melt crucible to form the liquid melt. The apparatus also includes at least one capillary conveyor in fluid communication with the melt crucible and the growth crucible to transport the liquid melt from the melt crucible to the growth crucible.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/694,226, filed Nov. 25, 2019, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE DISCLOSURE Field

The present disclosure generally relates to systems and methods forgrowing crystals from a melt and, more particularly, to systems andmethods for growing crystals where the melt is continuously replenishedduring crystal growth.

Description of Related Art

Single crystal sapphire is the material of choice for opticalapplications involving high temperature, shock loading, and abrasion dueto its optical properties, high strength, and abrasion and thermal shockresistance. Various methods have been developed for growing singlecrystal sapphire including the heat exchanger method, Edge-definedFilm-fed Growth (EFG) techniques, and Czochralski (Cz) techniques.

A conventional crystal growth system requires a batch-basedconfiguration where a single crucible is loaded with raw material. Thisraw material is melted into a single batch of liquid melt which is thendrawn through the die during the crystal growth process. The amount ofmaterial in the melt determines the maximum crystal size, and the dropin the melt level as the solid crystal is formed has to be accounted forin the process design.

Systems have been designed for continuous replenishment of the melt. Inone example, a segmented crucible with an inner and an outer area isprovided. Solid material is dropped into the outer area and melted byouter heaters before flowing into the inner area to supply the crystalgrowth. Such a system suffers from various deficiencies. For instance,such a segmented crucible design requires the superheated area formelting the solid material to be relatively close to the crystal growthregion, and the two cannot be decoupled thermally. This placessignificant limitations on the process window for successful crystalgrowth. In addition, the dropping of the solid material into the outerarea causes disturbance in the overall liquid level which can lead tothe loss of a single crystal structure. Still further, the thermalgradients in the liquid melt cause complex and sometimes deleteriousconvection patterns to develop, which tend to increase the presence ofinclusions in the crystals (either voids or small pieces of unmeltedsolid or other foreign particles). Finally, such segmented crucibles aretypically complicated and quite expensive to make.

SUMMARY OF THE DISCLOSURE

In accordance with one aspect of the present disclosure, provided is anapparatus for growing a crystal. The apparatus comprises a growthchamber and a melt chamber thermally isolated from the growth chamber.The growth chamber comprises: a growth crucible configured to contain aliquid melt; and a die located in the growth crucible, the die having adie opening and one or more capillaries extending from within the growthcrucible toward the die opening. The melt chamber comprises: a meltcrucible configured to receive feedstock material; and at least oneheating element positioned within the melt chamber relative to the meltcrucible to melt the feedstock material within the melt crucible to formthe liquid melt. The apparatus also comprises at least one capillaryconveyor in fluid communication with the melt crucible and the growthcrucible to transport the liquid melt from the melt crucible to thegrowth crucible.

In accordance with another aspect of the present disclosure, a method ofgrowing a crystal comprises: providing a growth crucible having a dielocated therein; providing a melt crucible thermally isolated from thegrowth crucible; melting feedstock material provided within the meltcrucible to form a liquid melt; and transporting the liquid melt fromthe melt crucible to the growth crucible with at least one capillaryconveyor provided in fluid communication with the melt crucible and thegrowth crucible.

These and other features and characteristics of the apparatus of thepresent disclosure, as well as the methods of operation and functions ofthe related elements of structures and the combination of parts andeconomies of manufacture, will become more apparent upon considerationof the following description and the appended claims with reference tothe accompanying drawings, all of which form a part of thisspecification, wherein like reference numerals designate correspondingparts in the various figures. It is to be expressly understood, however,that the drawings are for the purpose of illustration and descriptiononly and are not intended as a definition of the limits of the device ofthe present disclosure. As used in the specification and the claims, thesingular form of “a”, “an”, and “the” include plural referents unlessthe context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an apparatus for growing acrystal in accordance with the present disclosure;

FIG. 2 is a schematic cross-sectional view of a portion of the growthcrucible of the apparatus for growing a crystal of FIG. 1;

FIG. 3 is another schematic cross-sectional view of the portion of thegrowth crucible of FIG. 1;

FIG. 4A is a schematic cross-sectional view of the growth crucible ofFIG. 3 illustrating a crystal being grown;

FIG. 4B is a portion of the schematic cross-sectional view of FIG. 2enlarged for magnification purposes;

FIG. 5 is an enlarged perspective view of a capillary conveyor of theapparatus for growing a crystal of FIG. 1;

FIG. 6 is an enlarged top view of a portion of the capillary conveyor ofFIG. 5 illustrating the details of a cone-shaped element of thecapillary conveyor; and

FIG. 7 is an enlarged top view of a portion of the capillary conveyor ofFIG. 5 illustrating an alternative example of the cone-shaped element.

DESCRIPTION OF THE INVENTION

For purposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “lateral”,“longitudinal”, and derivatives thereof shall relate to the invention asit is oriented in the drawing figures. However, it is to be understoodthat the invention may assume alternative variations and step sequences,except where expressly specified to the contrary. It is also to beunderstood that the specific devices and processes illustrated in theattached drawings, and described in the following specification, aresimply exemplary embodiments of the invention. Hence, specificdimensions and other physical characteristics related to the embodimentsdisclosed herein are not to be considered as limiting.

Unless otherwise indicated, all ranges or ratios disclosed herein are tobe understood to encompass any and all subranges or subratios subsumedtherein. For example, a stated range or ratio of “1 to 10” should beconsidered to include any and all subranges between (and inclusive of)the minimum value of 1 and the maximum value of 10; that is, allsubranges or subratios beginning with a minimum value of 1 or more andending with a maximum value of 10 or less, such as but not limited to, 1to 6.1, 3.5 to 7.8, and 5.5 to 10.

With reference to FIG. 1, an apparatus for growing a crystal, generallydenoted as reference numeral 1, comprises a growth chamber 3 and a meltchamber 5 thermally isolated from the growth chamber 3. In one example,the growth chamber 3 and the melt chamber 5 are positioned side-by-sideas shown in FIG. 1. Each chamber 3, 5, is surrounded by an insulationhousing 7 with an insulation wall 9 thermally isolating the growthchamber 3 from the melt chamber 5.

The growth chamber 3, shown in a cross-sectional cut with growth dieplates 21 extending into the page, comprises: a growth crucible 11configured to contain a liquid melt 13 of material to be crystallized;and one or more dies 15 positioned within the growth crucible 11. WhileFIG. 1 illustrates that one die 15 may be positioned within the growthcrucible 11, this is not to be construed as limiting the presentdisclosure as any suitable number of dies 15 may be utilized. Die 15includes a die opening 17 and one or more capillaries 19 extending fromwithin the growth crucible 11 toward the die opening 17. A seed crystalmoving mechanism (not shown) is also provided in the growth chamber 3 tomove the seed crystal(s) away from the die 15 as the crystal is forming.

With reference to FIGS. 2, 3, 4A, and 4B, and with continued referenceto FIG. 1, additional details of the growth crucible 11 and die 15 arediscussed. For ease of explanation, FIGS. 2, 3, 4A, and 4B onlyillustrate one die 15 positioned within the growth crucible 11. Inaddition, the description hereinafter is related to an EFG technique forcrystal growth. However, this is not to be construed as limiting thepresent disclosure, as the principles disclosed herein with regard tocontinuous replenishment of a crucible are applicable to other crystalgrowth techniques such as Czochralski (Cz) techniques. As discussedabove, the growth crucible 11 contains a liquid melt 13 of the materialto be crystallized, such as Al₂O₃ powder. One or more heaters (notshown) may be positioned to supply heat to the growth crucible 11.

In general, the die material is selected such that it is chemicallyinert with respect to the liquid melt, but is wetted by the liquid melt.Wetting is indicated by the formation of a meniscus of liquid melt atthe top of die. The size (i.e., height) and shape of the meniscus isdetermined by the surface tension between the molten material and thesolid comprising the die, as well as the density of the liquid melt andgravity. The growth interface, where the crystal is formed, is at thetop surface of the die. Liquid melt is delivered to the top surface ofthe die by capillary flow of liquid rising through the capillaries 19 inthe die 15. The die 15 includes two adjacent rectangular die plates 21extending in and out of the plane of the drawing. The die plates 21 aretypically separated by spacers (not shown) so that the die plates 21 areonly separated by a small distance, typically from 0.05 cm to 0.3 cm,which corresponds to the width of the capillary 19. The die opening 17is defined as the distance from the outer edge of one die plate 21 tothe outer edge of the other, and determines the thickness of thecrystal. The arrangements of the die plates 21 result in capillaries 19through which the melt flows from the growth crucible 11 to the dieopening 17. The melt then flows to the die opening 17 to form a thinwetting layer of melted material (the meniscus 23) having boundaries atthe outer edges of the die plates 21 (see FIGS. 4A and 4B). The crystalgrowth occurs at the solid/liquid interface at the upper surface of thethin wetting layer of melted material, with the resulting crystal 25having a thickness spanning the die opening 17 (as defined above). Aftera spreading process from the initial seed crystal, the final width ofthe crystal is determined by the length of the die (in and out of theplane of the drawing). The height is determined by how long and how fastthe crystal pulling process proceeds.=With specific reference to FIG. 3,the height (h) above the surface of the liquid melt that the liquid willrise in the capillaries 19 is determined by the dimensions of thecapillaries 19, the surface tension between the melt and the diematerial, the density of the melt, and gravity.

A seed crystal (not shown), cut from a previously grown crystal, isbrought into contact with the top of the die. Due to the high thermalgradient at the die, portions of the seed melt on contact with the die,and the thin liquid layer eventually merges with the capillary liquid.As the seed is pushed further down, eventually the solid bottom edge ofthe seed becomes conformal with the die surface 15. Once this intimatewetted contact is established and a full meniscus 23 is present, theseed can be pulled up, away from the die 15 at a predetermined speed. Asthe seed crystal is withdrawn, the melt material crystallizes onto theseed crystal. While initial seed contact is made at only one point onthe die (e.g. the center), with proper thermal management the meniscus23 spreads across the length of the die 15 (in and out of the plane ofthe drawing). The spreading is caused by the progressive lateralfreezing of the meniscus at the edges of the crystal (not shown incross-section). The meniscus spreads until it reaches the edge of thedie 15. Various physical principles prevent the meniscus from goingbeyond the edge of the die. As long as the angle formed by the edges ofthe die 15 is 90° or less, the meniscus 23 is constrained by the edge ofthe die 15. As the seed crystal is raised, the meniscus 23 spreadsacross the surface of the die 15, and the crystal spreads as well(quickly in the left/right directions, slowly in/out of plane).Temperature adjustments to the growth crucible 11, to the cavity intowhich the seed is pulled, and/or adjustments to the seed lift rate aremade as required to promote the spread of the crystal across the surfaceof the die 15 and to control its growth rate. When the crystal reachesthe shape defined by the edges of the die 15 in both dimensions, itsfinal form is set by the edges of the die 15.

At that point, a steady state equilibrium is established, and, as longas liquid continues to be delivered to the surface of the die 15 bycapillary action from the melt 13, a crystal of constant cross-sectiondefined by the top surface of the die 15 is grown. The heaters used tomaintain the growth crucible 11 and the inside of the growth chamber 3at the appropriate temperature are chosen by factors such as the size ofthe crucible/crystal, the geometry of the crucible (round, rectangular),and the power requirements. In some instances, induction heating may beemployed. Other systems utilize resistance heating elements. A hybridapproach may also be utilized in which induction coils are used to heata susceptor (usually graphite in the case of sapphire growth), wherebythe susceptor then acts like a resistance heating element that radiatesheat to the crucible.

In addition, the apparatus may also include an afterheater (not shown).The afterheater sits above the growth crucible 11 and provides anenvironment in which the crystal's temperature drops gradually from thefreezing point to some intermediate value below the freezing point butsignificantly above room temperature, such as around 1800° C. in thecase of sapphire crystal growth. Then, once growth is completed and thecrystal has been separated from the die, it can be cooled down to roomtemperature in a controlled fashion. The afterheater can be eitheractive or passive. A passive afterheater receives heat radiated orconducted from the hot zone containing the growth crucible 11. An activeafterheater has its own heat source. This can be controlled separatelyfrom the main heat source heating the growth crucible 11 and thus beindependent, or it can utilize the same power supply, in which case itstime-temperature profile tracks that of the growth crucible 11 albeit ata lower value.

Returning to FIG. 1, the melt chamber 5 of the apparatus comprises amelt crucible 27 configured to receive feedstock material 29, such asAl₂O₃ powder if sapphire single crystal is intended to be formed by theapparatus. The feedstock material 29 may be provided to the meltcrucible 27 by any suitable conveying mechanism such as, but not limitedto, a ramp 31. Other conveying mechanisms include a vibratory feeder, arotary feeder, a tilting hopper, or an air-pressure modulated feeder.The feed rate at which the feedstock material 29 is added to the meltcrucible 27 is carefully monitored using appropriate sensors (notshown). The feed rate of the feedstock material 29 into the meltcrucible 27 must be controlled to at least roughly match the rate ofcrystal growth using appropriate control mechanisms based on feedbackfrom the sensors.

In addition, the melt chamber 5 is provided with one or more heatingelements 32 positioned therein relative to the melt crucible 27 to meltthe feedstock material 29 within the melt crucible 27 to form the liquidmelt 13. The heating element 32 may be an inductive heating element, aresistive heating element, or a hybrid of both inductive and resistiveheating elements. In one example, the melt chamber 5 may be designedusing induction heating to maximize efficiency while the growth chamber3 may utilize resistive heating.

In some examples, the melt crucible 27 may have a much smaller or muchlarger volume than the growth crucible 11. For instance, the meltcrucible may have a volume of about 1,500 cm³ to about 100,000 cm³ andthe growth crucible may have a volume of about 13,000 cm³ to about50,000 cm³. The use of a larger melt crucible 27 provides additionalstability to the process because it requires a significant amount ofmaterial to be needed to change the net liquid melt level.

The isolation of the melt crucible 27 from the growth crucible 11provides several advantages over existing systems for growing crystals.For instance, the delivery of feedstock material 29 into the meltcrucible 27 may create disturbances in temperature or fluid flow thereinwhich would negatively affect the crystal if such disturbances occurredin the growth crucible 11. However, since the melt crucible 27 iscompletely isolated from the growth crucible 11, such disturbances donot affect crystal growth. In addition, the heating elements 32 in themelt chamber 5 can be run at a high rate in order to drive efficientmelting without affecting the temperature distribution in the growthchamber 3.

The apparatus 1 also comprises a capillary conveyor 33 in fluidcommunication with the melt crucible 27 and the growth crucible 11 totransport the liquid melt 13 from the melt crucible 27 to the growthcrucible 11 via capillary action caused by depletion of the liquid melt13 in the growth crucible 11 as the crystal 25 is grown such that aliquid melt level 34 in the melt crucible 27 is the same as the liquidmelt level in the growth crucible 11. While only a single capillaryconveyor 33 is illustrated in FIG. 1, this is not to be construed aslimiting the present disclosure as two or more capillary conveyors maybe utilized to transport the liquid melt 13 from the melt crucible 27 tothe growth crucible 11. Similarly, in order to control the cost of themelting crucible, several small melting crucibles might be used in placeof a large one, where each is connected via capillary channels to theothers.

With reference to FIG. 5, and with continued reference to FIG. 1, thecapillary conveyor 33 is formed from a pair of flat metallic plates 35positioned a distance apart forming a capillary channel 36 having acontrolled gap-width therebetween. The width of the capillary channel 36must be sufficiently small to assure the natural wetting of the liquidmelt 13 from the liquid melt level 34 within the melt crucible 27 to thetop of the capillary conveyor 33. In some examples, this width may beabout 0.05 cm to about 0.3 cm apart. The pair of flat metallic plates 35may each have an upside down U-shape with a first leg 37 of the upsidedown U-shape being positioned within the melt crucible 27, a second leg38 of the upside down U-shape being positioned within the growthcrucible 11, and a portion 39 extending between the first leg 37 and thesecond leg 38. The portion 39 extending between the first leg 37 and thesecond leg 38 optionally includes at least one rotary flow element, suchas cone-shaped element 41, provided in a path of the liquid melt. Therotary flow element comprises a cavity between the two plates 35 with ageometry conducive to rotational flow. Accordingly, while a cone-shapedelement 41 is illustrated in FIGS. 5-7 and will be discussedhereinafter, this is not to be construed as limiting the presentdisclosure as any suitable cavity having a geometry configured to createa rotational flow may be utilized as the rotary flow element.

With reference to FIG. 1 and with continued reference to FIGS. 5-7, theentire length of the capillary conveyor 33 must be maintainedcomfortably above the melting temperature of the feedstock material.Accordingly, appropriate insulation must be provided surrounding thecapillary conveyor 33. In addition, heaters (not shown) may be providednear or around the capillary conveyor 33. However, it should be notedthat the passage of the capillary conveyor 33 into the growth chamber 3is configured to thermally isolate the melt chamber 5 and the growthchamber 3 as much as possible while still maintaining capillary conveyor33 above the melting temperature of the feedstock material.

One of the primary problems in crystal growth is the presence ofmicroscopic bubbles. The presence of such bubbles is deleterious to useas an optical material, causing scattering of light. Small bubbles arebasically stable in a sapphire melt and, for EFG crystal growth, cantravel up through the capillaries and into the grown material. Thecone-shaped element 41 acts as a bubble trap in the capillary conveyor33 such that bubbles are eliminated prior to reaching the growthcrucible 11. With reference to FIGS. 6 and 7, the capillary channel 36of the capillary conveyor 33 includes the at least one cone-shapedelement 41 through which almost all of the liquid melt would pass in thedirection of arrows F. The cone-shaped element 41 creates a vortexconvection due to its geometry and the flow passing through. Such avortex concentrates bubbles 43 towards the center as denoted by arrows Abecause these bubbles 43 are less dense than the surrounding area. Thebubbles 43 are thereby trapped in the volume of the cone-shaped element41 and do not flow through to the second half of the capillary conveyor33 that leads to the growth crucible 11. Natural buoyancy pushes thebubbles 43 towards the top of the cone-shaped element 41, where thecentral tendency of the centripetal force is greatest. In the center,the bubbles 43 agglomerate and eventually “pop”, leaving the liquid meltaltogether.

In addition, the cone-shaped element 41 may comprise at least oneportion, such as fins 45 (see FIG. 6) or arc-shaped portions 47 (seeFIG. 7), extending from an inner surface thereof. Such portions causethe bubbles 43 to move to a central area of the cone-shaped element 41.More specifically, such portions help to create circular flow within thecone-shaped element 41, thereby guiding the bubbles 43 to the center ofthe cone-shaped element 41.

In operation, the apparatus 1 functions as follows. First, feedstockmaterial 29 is provided to the melt crucible 27 by any suitableconveying mechanism where the feedstock material 29 is melted to formthe liquid melt 13 as described hereinabove. Thereafter, the liquid melt13 from the melt crucible 27 is transported to the growth crucible 11with capillary conveyor 33 provided in fluid communication with the meltcrucible 27 and the growth crucible 11. As such, the growth crucible 11contains the liquid melt 13, and the dies 15 are mounted partly withinthe growth crucible 11. As described hereinabove, the dies 15 have oneor more capillaries 19 extending from within the growth crucible 11 tothe die opening 17. Capillary action is used to draw liquid melt 13 fromthe growth crucible 11 through the one or more capillaries 19 and ontothe surface of the die openings 17 of each of the dies 15. Next, a seedcrystal is inserted into the liquid melt 13 on the surface of the dieopening, and then the seed crystal is pulled away from the surface ofthe die opening 17 in a controlled manner to grow a crystal. After thecrystal is formed, the growth process is stopped by quickly removing thepanel(s) from the die, breaking the liquid contact, and the crystal canbe removed for further processing (such as grinding, polishing, lapping,or the removal of bulk material).

Accordingly, the apparatus and method of the present disclosure offerseveral advantages over the conventional batch-based approach. First,the size of the growth crucible 11 and the initial volume of liquid melt13 provided in the growth crucible 11 no longer limits the size of thefinal crystal. In many cases, this allows the size of the growthcrucible 11 to be decreased which reduces crucible costs and increasesthe process safety by decreasing the size of potential melt spills. Atthe same time, the final crystal size can actually be larger than inconventional batch processes since crucible size is often a limitationboth in cost and in space in the apparatus. The resulting process yieldis higher because less time is spent in turn-overs. The overallthroughput also increases because the time that is normally spentmelting the feedstock material is put in parallel to the growth process.A small amount is melted at the beginning, and the remainder is meltedon-the-fly during the growth process. Finally, the process is typicallymore stable in continuous replenishment because the melt level can bemaintained at a constant value throughout the process. In EFGtechniques, this means that the supply of material through the capillarywill remain constant and not decrease as the crystal is pulled. In Cztechniques, this means that the crucible does not need to be raised inorder to maintain a steady melt level in the furnace, and that theconvective flow in the melt can be kept in a steady state.

While specific embodiments of the device of the present disclosure havebeen described in detail, it will be appreciated by those skilled in theart that various modifications and alternatives to those details couldbe developed in light of the overall teachings of the disclosure.Accordingly, the particular arrangements disclosed are meant to beillustrative only and not limiting as to the scope of the device of thepresent disclosure which is to be given the full breadth of the claimsappended and any and all equivalents thereof.

The invention claimed is:
 1. An apparatus for growing a crystal, theapparatus comprising: a growth chamber having: a growth crucibleconfigured to contain a liquid melt; and a die located above the growthcrucible, the die having a die opening and one or more capillariesextending from within the growth crucible toward the die opening; a meltchamber thermally isolated from the growth chamber, the melt chamberhaving: a melt crucible configured to receive feedstock material; and atleast one heating element positioned within the melt chamber relative tothe melt crucible to melt the feedstock material within the meltcrucible to form the liquid melt; and at least one capillary conveyor influid communication with the melt crucible and the growth crucible totransport the liquid melt from the melt crucible to the growth crucible.2. The apparatus of claim 1, wherein the melt chamber is thermallyisolated from the growth chamber by insulation.
 3. The apparatus ofclaim 1, further comprising a feedstock conveyor in communication withthe melt crucible configured to supply the feedstock material to themelt crucible.
 4. The apparatus of claim 1, wherein the melt cruciblehas a larger volume than the growth crucible.
 5. The apparatus of claim4, wherein the melt crucible has a volume of about 1,500 cm³ to about100,000 cm³ and the growth crucible has a volume of about 13,000 cm³ toabout 50,000 cm³.
 6. The apparatus of claim 1, wherein the capillaryconveyor is positioned to transport liquid melt from the melt crucibleto the growth crucible via capillary action caused by a higher liquidlevel in the melt crucible, either due to depletion of the liquid meltin the growth crucible as a crystal is grown or addition of feedstock toraise the liquid level in the melt crucible.
 7. The apparatus of claim1, wherein the at least one capillary conveyor is formed from a pair ofmetallic plates positioned a distance apart.
 8. The apparatus of claim7, wherein the metallic plates are positioned a distance of about 0.05cm to about 0.3 cm apart.
 9. The apparatus of claim 7, wherein the pairof metallic plates each have an upside down U-shape with a first leg ofthe upside down U-shape being positioned within the melt crucible, asecond leg of the upside down U-shape being positioned within the growthcrucible, and a portion extending between the first leg and the secondleg.
 10. The apparatus of claim 9, wherein the portion extending betweenthe first leg and the second leg comprises at least one rotary flowelement provided in a path of the liquid melt.
 11. The apparatus ofclaim 10, wherein the at least one rotary flow element comprises atleast one portion extending from an inner surface thereof configured tocause bubbles to move to a central area of the at least one rotary flowelement.
 12. The apparatus of claim 10, wherein the rotary flow elementcomprises a cavity between the two metallic plates with a geometryconducive to rotational flow.
 13. The apparatus of claim 10, wherein therotary flow element is cone shaped.